The treatment of wounds has become a highly developed area of scientific and commercial investigation because increased rates of healing reduces healthcare costs and decreases the risk of complications due to secondary infections. It is currently believed that healing is related to the degree of injury, the immunological and nutritional status of the host, contamination of the wound, the maintenance of the moisture level, pH and oxygen tension of the wound surface, and the electrical parameters of the wound site in relation to the surrounding intact, uninjured tissue. In particular, regeneration in amphibians and fracture healing in mammals are associated with complex changes in the local direct current (DC) electric field. It is believed that the electric field gradually returns to normal pre-injury levels as the injury heals. Conversely, failure of the normal healing process, for example as in fracture non-unions, is associated with the absence of appropriate electrical signals at the site of the injury.
There have been numerous studies conducted on wound healing in amphibians because their rate of healing is significantly greater than that of mammals. Wound healing in mammalian skin occurs over days or even weeks, with epithelial cell migration rates ranging from 7 (dry wound) to 20 (wet wound) micrometers/hour. Amphibian skin wounds heal within hours, with epithelial cell migration rates ranging from 60 to more than 600 micrometers/hr. The expedited rates of healing in amphibian skin may be partially explained by the aqueous environment that bathes the outer surface of the epithelium. Amphibian wounds in an aqueous environment are provided with the appropriate ions to reestablish the electrical potential on the surface of the wound as well as provided with an environment favorable to cell migration and reproduction.
It is generally recognized that dry wounds in mammals heal more slowly than wounds that are kept moist by occlusive dressings. Keeping the epidermis surrounding a wound and the wound itself moist stimulates the wound to close. Wound dressings have been designed to retain moisture from the exudates produced by the wound and function by preventing evaporation of fluid. Wounds that are dry and lack production of exudate must depend upon the moisture within a self contained wound dressing. If the wound dressing dries out, the needed moisture level for optimum wound healing will not be maintained and the dressing will stick to the wound surface and cause disruption of cellular processes. The lack of moisture often results in the formation of an eschar or scab, and a general slowing of the wound healing process.
Wounds that produce an extensive amount of moisture are thought to create another problem called skin maceration. Skin maceration is a softening of the skin or wearing away of the skin as a result of continual exposure to bodily fluids or moisture. It is known to cause a breakdown of the cornified epithelium, thereby reducing the physical microbial barrier function as well as the moisture regulation function of the epidermis. With a reduction of the microbial barrier function, the wound surface has a significantly greater risk of contamination by pathogenic microbes from the surrounding environment. Therefore, it is common practice to design wound dressings to reduce or prevent skin maceration by wicking away wound fluids and storing the fluids in absorbent layers.
A common practice in the treatment of wounds is the application of impermeable backing sheets to a wound dressing. The backing sheet functions as a moisture retention layer as well as a physical barrier to prevent microbial penetration. The backing sheet typically consists of a material with specified moisture vapor transmission rates (MVTR) and provides control of the rate of evaporation of moisture from the absorbent layer. Therefore, the backing sheet is generally impervious to liquid.
There are a variety of venting systems that can be contained within the dressing structure for the purpose of directing wound exudates via specific pathways to provide a controlled leakage of fluids from the would surface to a contained absorbent layer. For example, in certain perforated films, the perforations are sufficient to permit wound exudates to diffuse through the film at a rate that precludes pooling on the wound surface, which is a common cause of maceration. These dressings must be removed when they become saturated with exudates.
While there are numerous dressings designed to retain the moisture content of wounds, there are still many areas of inefficiency in current treatment methods. For example, these dressings are only effective for moist wounds and do not provide any significant benefit for dry wounds. Wounds vary significantly in the amount of exudates or moisture produced throughout the healing cycle. In order to maintain an effective level of moisture it is necessary to continually change the dressings as the absorbent component reaches maximum capacity. Conversely, it is necessary to remove the dressings and add fluid to dry wounds, then replace the dressings. In either situation, removal of the dressing can cause disruption of the cellular process of the wound and increase the risk of contamination by microbes. Furthermore, it is necessary to change the types of dressings throughout the healing process of the wound as the moisture content changes.
Besides the effect of moisture on wound healing, microbial growth at the site of injury has a great effect on healing. In normal skin, a microbial barrier is created by the cornified epithelium. Wounds cause destruction of the cornified epithelium as well as deeper layers thereto, and the loss of the natural anti-microbial barrier.
The presence of microbial species at the wound site creates a bioburden that can retard the healing process. As the bioburden of the wound decreases to bacterial counts less than 103 CFU/ml, wound healing is enhanced. Treatment of wounds typically involves preventing contamination by pathogenic microbes from the external environment as well as reducing the microbial bioburden of the wound.
While there are scores of antibacterial and antifungal agents that can be used to treat wounds, the anti-microbial and antifungal properties of silver have been of particular interest. However, the effectiveness of silver as an anti-microbial agent is at least partly determined by the delivery system. Most silver compounds that dissociate readily and produce large numbers of free silver ions are highly toxic to mammalian tissues. Less-toxic compounds, including silver sulfadiazine cream, widely used in the treatment of burns, do not dissociate readily and therefore do not release large numbers of silver ions. Therefore, these compounds must be re-applied frequently to maintain their clinical efficacy.
Silver has been used in the construction of wound dressings to actively or passively release metallic silver particles or silver ions into the wound. Active release of silver ions require the presence of an electrical potential that actively drives silver ions from a source into the wound dressing or wound itself. This has been accomplished with a battery or other power source known to those skilled in the art. Passive release of silver ions is dependent upon the solubility of silver in aqueous solutions. The passive release of silver ions has been called the oligodynamic release process and includes the passive dissolution of silver into a solution.
The anti-microbial efficiency of metallic silver or silver ions is dependent upon the microbe coming into direct contact with the surface of the metallic silver or coming into contact with a released silver ion. Therefore, the total surface area of metallic silver and the number of silver ions released is directly related to the level of anti-microbial activity. Various methods have been used to create mechanisms for metallic ion transfer.
For example, the vacuum vapor deposition technique has been utilized in the construction of wound dressings to plate metallic silver and silver salts onto a variety of substrates. The vacuum vapor deposition technique has been modified so as to create “atomic disorder” of the plated silver that has been reported to enhance the anti-microbial effect by allowing the release of nanocrystaline particles of metallic silver. However, the technique provides a flat plating pattern and does not uniformly coat the entire three-dimensional surface of fibers.
Another mechanism used for passive release of silver ions and particles from a wound dressing includes imbedding or placing silver particles of varying sizes in a variety of substrates. Finely divided metallic silver in collagen has been incorporated into surgical dressings of reconstituted collagen foam laminated to a thick continuous layer of inert polymer. This does not allow for direct contact of the maximum number of ions with the wound.
When connected to a voltage source, a metal anode and a return electrode have been used as a means to deliver silver ions iontophoretically to a wound or within a wound dressing. Electrically conductive silver-impregnated meshes, including silver-protein colloids, have been disclosed with current densities as low as 10 μA/mm2. This requires an external power source and stationary equipment and is cumbersome for the patient.
Silver foils have been incorporated into wound dressings as a means of supplying silver ions as an anti-microbial agent, as well as acting as an electrode for dispensing medications. In addition, silver has been fabricated into devices that incorporate a means of applying a therapeutic voltage to the wound. Foils do not provide for circulation of air, and are limited in surface area.
Compounds that slowly release silver into the wound environment have been disclosed in substances such as water soluble glass, phosphorus pentoxide and silver oxide. The silver impregnated glass may be in the form of a powder, granules, or woven into a dressing. The water soluble glass releases silver secondary to the dissolution of the glass. Such compositions have a high volume resistance and very poor conductivity.
Regardless of whether silver is provided in the form of silver ions or as a topical composition (silver nitrate solution, silver sulfadiazine cream, or the like), its beneficial effects are manifested primarily at the treated surface and immediately adjacent tissues, and are limited by the achievable tissue concentration of silver ions. Despite the availability of numerous techniques for the delivery of silver and silver compounds in vitro and in vivo, there remains a need for a delivery system that is capable of supplying clinically useful concentrations of silver ions to a treatment site without the need for adjuvant electrical stimulation.
None of the available metallic ion treatment devices provide an efficient and convenient means to restore the homeostatic electromagnetic environment for areas of wounds. They also do not provide for maximum surface area for release of metallic ions. In addition, the prior art does not address the need to regulate the moisture content of a wound without manually changing the dressings, or applying liquids or medicants. This is true in part because of the belief that a wound dressing must serve as a microbial barrier and prevent the movement of fluids from the wound exudates. The currently available treatments for wounds prevent microbial contamination by providing a physical barrier which must be manipulated and interrupted as part of the treatment process. Such activities allow for microbe contamination and interrupt the healing process.
It is believed that wound healing occurs with maximum speed and is efficiency when the wound is maintained in a moist condition without excessive wetness or dryness. Wounds have variable hydration needs based upon the type of wound and the phase of healing. There are many types of wound dressings on the market to meet the differing needs of different types of wounds, however, none provide for the regulation of the fluid content of the wound.
What is needed is a means for treating wounds that addresses the above problems, and provides a functional anti-microbial barrier, allows for regulation of the moisture content of the wound, and aids in maintaining the transepithelial potential across the epithelium.